The uranium oxide systems have been heavily studied due to uranium use in commercial fuel fabrication as well as other applications such desulfurization, selective reduction of nitrogen oxide, and various applications involving reaction with organic compounds. In aqueous solutions, the U(VI) oxidation state predominates the chemistry through stabilizing trans-oxo ligands forming the uranyl cation UO22+. In the solid state, there is minimal energy difference between the U(IV) and U(VI) oxidation states and as a result, both of these oxidation states are present in triuranium octoxide (U3O8), which is the most kinetically and thermodynamically stable form of uranium and occurs naturally in the mineral pitchblende.
Particularly in the case of uranium-based fuel, uranium dioxide (UO2) is the preferred form of uranium due to the high fissile density of the U(IV) oxidation state in this compound. Traditionally, to form uranium dioxide, a uranium-bearing material such as uranyl nitrate hexahydrate, uranyl oxalate, or uranium peroxide dihydrate is first oxidized to form uranium trioxide (UO3) followed by reduction at high temperature (e.g., about 700° C.) with hydrogen. Powders of the uranium dioxide can then be further treated as desired. For instance uranium dioxide powders can be enriched in 235U, mixed with any additional materials, such as binders, pore forming materials, or other oxides, pressed to compact and form a green body, sintered, and shaped to form fuel pellets. Small dimension particles are generally preferred for the powders to improve flowability, compaction, and sinterability of the materials.
Unfortunately, the uranium dioxide particle formation process is both expensive due to the high temperature requirements and dangerous due to the use of hydrogen gas.
Ionic liquids are a class of solvents that have been found useful as solvents in low temperature synthesis processes. These materials have produced unexpected results as they have been found to provide a solvation environment that is completely different from organic solvents that have been utilized in traditional synthesis. Ionic liquid solvents have also been examined for use in dissolving spent nuclear fuel and fission products so as to oxidize the metals as described by Fields, et al. in U.S. Pat. No. 6,379,634.
What are needed in the art are methods for formation of uranium oxides such as uranium dioxide that can be carried out at low temperature and by use of reducing agents that do not present the dangers of currently used hydrogen. Methods that can form nano-sized morphologies for use in a variety of applications would also be of benefit in the art.